CCT stands for Correlated Color Temperature. It defines color, not brightness of a light source and is measured in degrees Kelvin. A low CCT is in the amber end of the color spectrum while a high CCT is in the blue-white end of the spectrum. For reference purposes, a standard incandescent light is about 3000K while the bright white xenon headlights of some newer cars are 4500K.
There is a national movement to either limit or eliminate high CCT outdoor lighting. The reasons are twofold. First, in June 2016, the American Medical Association (AMA) adopted a groundbreaking report that affirms known and suspected impacts to human health and the environment caused by high CCT outdoor lighting. The AMA therefore recommends all outdoor lighting be 3000K or less because higher CCT lighting causes sleep disruption (which leads to obesity, depression, diabetes, and more) and disability glare.
Second, outdoor lighting above 3000K CCT causes light scattering in the atmosphere, which results in graying of the sky and a significant reduction in the ability to see stars and enjoy the wonders of our nighttime universe. See chart below.
Because of the above concerns, many communities around the country have updated their outdoor lighting ordinances to require all new outdoor lighting to have a CCT of 3000K or less. They have also taken steps to similarly reduce the CCT for existing outdoor lighting.
LEDs represent the latest in energy efficient lighting. When first introduced, the standard CCT of LEDs was 4000K or higher, which produces a glaring, very bright white light. However, it is important to know that LEDs are now available with a CCT of 3000K or even lower and should be chosen when being purchased for outdoor lighting.
